This invention relates to the measurement of characteristics of a.c. waveforms.
In the field of electronic circuitry there is often a requirement for the shape of a pulse waveform to be measured so that it can be analyzed to assess whether it meets certain criteria. The measured pulse waveform can be compared with a standard mask to see whether it meets a prescribed specification. This comparison may include the measurement of the pulse width, the rise time, the fall time and any overshoot or undershoot.
A known technique for measuring the shape of a high speed a.c. waveform makes use of two circuits each with a programmable delay. One such circuit is used to trigger a device under test to produce an output pulse which is fed to a comparator. The other circuit is used to enable the comparator and its delay setting is adjusted to the point where the level of output of the device under test equals a threshold signal applied to the comparator from a digital-to-analogue converter. By varying the threshold level and adjusting the delay of the second circuit it is possible to produce a point-by-point reconstruction of the a.c. waveform output from the device under test. The circuit operates in response to clock pulses applied to the two circuits with the programmable delay.